This invention relates to a process for producing a 3-layer co-extruded biaxially oriented polypropylene (hereinafter as BOPP) synthetic paper of thickness 25xcx9c250 xcexcm. More particularly, the invention relates to a process for producing a 3-layer biaxially oriented PP synthetic paper by means of 3-layer co-extrusion wherein two different PP resin compositions are separately extruded by one primary and two secondary extruders first, and then are co-flowed by a same T-die to form a 3-layer coating sheet, and through cooling, biaxial orientation, corona treatment and winding to form a 3-layer co-extrusion synthetic paper of thickness 25xcx9c250 xcexcm. The three layers structure of said 3-layer co-extrusion synthetic paper of thickness 25xcx9c250 xcexcm can be made into three kinds, such as: paper sheet layer/foamed intermediate layer/paper sheet layer with double side paper sheet layer, paper sheet layer/foamed intermediate layer/resin layer with single side paper sheet layer and resin layer/foamed intermediate layer/resin layer with double side high gloss resin layer.
Nowadays the paper made of nature pulp is replaced by the polyolefin synthetic paper mainly comprising the basic substrate layer in the intermediate layer called biaxial oriented polypropylene (BOPP), whereon the back surface is laminated with the uniaxially oriented polypropylene (UOPP) containing inorganic fine powder as the paper surface layer(such as the Japanese Patent Publication No. 40794/71, Japanese Laid-open Publication No. 141339/81, Japanese Laid-open Publication No. 118437/81 and Japanese Laid-open Publication No.87225/91).
The substrate layer used in the above-mentioned process between the longitudinal orientation device and lateral orientation device can be subjected to form paper sheet layer by using two extruders for casting the upper layer and lower layer. Paper sheet layer can merely be drawn in the lateral and uniaxial axis orientation, so its strength is lower. The inorganic powder added to enhance the paper sheet layer will apt to lose during printing processing, this causes the trouble to clean the printer. On the other hand, the drawing ratio of the paper sheet layer manufactured from single axis drawing (uniaxial orientation) may be different from the total drawing ratio of the substrate layer manufactured from biaxial orientation. When the synthetic paper is heated , the shrinkage ratio between the paper sheet layer and the substrate layer varies, this always cause the corrugated deformation in the synthetic paper.
For the stability of processing and the product quality, the production speed of two extruders used for casting the upper layer and lower layer to enhance the layer thickness to form the paper sheet layer will be more restricted. The width of the synthetic paper manufactured is lower than 6 meters., and the thickness of the single one paper sheet layer should be greater than 10 xcexcm (usually is 30 xcexcm). So the thickness of the synthetic paper product is always greater than 60 xcexcm, this may cause dispersity of the product thickness, and will deteriorate the print processing. As a whole, due to technical difficulty of processing., the production cost will be increased to limit the productivity, and can not be promoted to spread the application uses.
It is an object of this invention to provide a novel process for producing a 3-layer co-extrusion biaxially oriented polypropylene (BOPP) synthetic paper of thickness 25xcx9c250 xcexcm.
In view of the foregoing, this invention is intended to obviate the above-mentioned problems, and has for its object to provide a synthetic paper have a 3-layer structure, which is constructed by paper sheet layer or resin layer foamed intermediate layer paper sheet layer or resin layer. The PP resin composition used for foamed intermediate layer is extruded by a twin screw primary extruder with side feeding hopper. While the PP resin compositions used for paper sheet layer and/or resin layer are extruded by two twin screw secondary extruders with side feeding hopper. The above-mentioned extrudates are co-flowed by a same T-die to co-extrude and form a three layers coating sheet, and through cooling, biaxial orientation, corona treatment, and winding to form a 3-layer co-extrusion BOPP synthetic paper of double side, single side paper sheet layer and double side resin layer with high gloss.
The synthetic paper obtained by co-flowing the extrudate and then co-extruding to subject to biaxial orientation according to this invention, and the synthetic paper obtained by laminating the intermediate layer which is biaxially drawn with the paper sheet layer which is uniaxially drawn, are different whatever in structure and processing method. Meanwhile, the processing method according to this invention preferably uses resin compositions that comprises inorganic filler or not which can be extruded in the secondary extruder to form BOPP synthetic paper having paper sheet layer/foamed intermediate layer/paper sheet layer of double side paper sheet layer, BOPP synthetic paper having paper sheet layer/formed intermediately layer/resin layer of single side paper sheet layer, BOPP synthetic paper having resin layer/foamed intermediate layer/resin layer of double side high gloss resin layer. These synthetic papers can be used as the substrate layer for art paper grade synthetic paper.
There has been a long felt need for a synthetic paper which may be manufactured in larger width over 6 meters and smaller thickness below 60 xcexcm, and excellent physical properties in printability, pen-writing, packaging . . . etc.
The BOPP synthetic paper obtained according to this invention, due to the specific process and compositions, will make much difference with the synthetic paper obtained according to conventional prior art. The paper sheet layer of the above-mentioned conventional prior art which is obtained by casting melt resin composition whereon only after longitudinal and uniaxial orientation is a uniaxial orientation layer. The BOPP synthetic paper obtained according to this invention can replace natural pulp, can be used in various applications. The merits can be shown as follows:
1. The composition ingredients of said paper sheet layer is mainly comprised of polypropylene (PP), polyethylene (PE), titanium oxide and inorganic powder. The haze effect of this paper sheet layer is excellent, and the whiteness, hiding power (represented as opaqueness) are better than those of conventional paper.
2. The strength of said paper sheet layer obtained by biaxial orientation is excellent. The fixation of inorganic powder onto the surface of said paper sheet layer is fine enough not to cause powder fell while print processing.
3. 3-layer of the BOPP synthetic, such as: paper sheet layer or resin layer/foamed intermediate layer paper sheet layer or resin layer have the drawing ratio in itself, will be uniformly shrank, and the paper sheet layer or resin layer will not to cause corrugated deformation upon heating. The paper sheet layer and foamed intermediate layer of same drawing ratio will not easily delaminate.
4. The thickness of paper sheet layer and resin layer can be controlled in the range of 1 xcexcmxcx9c30xcexcm by the extrudate amount from secondary extruder (as the product thickness and application required).
5. The production speed is large to reach 3.5 tons/hr, and the width of the BOPP synthetic paper can reach to 8 meters. The total thickness of the BOPP synthetic paper can vary in the range of 25xcx9c250 xcexcm.
6. The 3-layer of the BOPP synthetic paper such as: paper sheet layer or resin layer foamed intermediate layer paper sheet layer or resin layer can be extruded by the twin screw extruder with side feeding hopper. The resin composition fed into the extruder can be uniformly blended, so the coating sheet can be controlled with xc2x12%.
7. To reduce the production cost for market competence.
The inorganic powder can be fed into the side feeding hopper in the powder state without pre-forming it into master batch, this can largely reduce the production cost. Furthermore, the master batch of inorganic powder blended with resin can also be used in the single screw extruder only if by effectively raising the L/D (length/diameter) of the screw to reach uniform blending effect.